These forums exist because Cryptic wants to get feedback from their customers so that they can improve their product/service. Threads filled with posts giving detailed explanations on why people don't like a certain thing or mission gives them the information that they need.

I would add that it is also a valuable resource to the player community as well. Many people post guides or walkthroughs to help those who have not run those things yet. It is also a place where players can share information on builds, gear or tactics and the like.